Contamination of soils with potentially toxic elements (PTEs) represents a significant environmental risk, particularly in areas affected by long-term mining and metallurgical activities. The aim of this study was to evaluate the effect of two Slovenian natural zeolites of different particle sizes and application rates on the availability of lead (Pb), cadmium (Cd), and zinc (Zn) in historically contaminated soils from the Upper Meža Valley, Slovenia. In a pot experiment, contaminated soil was amended with zeolites originating from Gornji Grad and Zaloška Gorica at application rates of 5, 10, and 20%. Uptake of PTEs by red beet (Beta vulgaris var. Conditiva) was assessed. At the end of the experiment, concentrations of Pb, Cd, and Zn were determined in soil and plant tissues. The results showed that zeolite amendments reduced the availability and plant uptake of PTEs, with stronger effects observed at higher application rates and for finely ground zeolite fractions. The greatest reductions in Pb, Cd, and Zn concentrations were achieved with the 20% zeolite treatments. On average, the Pb concentration in red beet leaves exceeded the maximum permissible threshold by a factor of 18, whereas in the tubers, it surpassed the limit 12-fold. Similarly, Cd concentration was even more pronounced, exceeding the regulatory threshold 68-fold in the leaves and 13,3-fold in the tubers. The study demonstrates the potential of Slovenian natural zeolites as an environmentally friendly amendment for immobilising PTEs in contaminated soils. However, zeolite application alone is insufficient to ensure safe crop production on heavily contaminated sites.
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